2. OX Tongue & Tripe** with roasted chili-peanut vinaigrette
4. Sliced Pork Belly* with chilli-garlic soy
19. Spicy Cucumber Salad*
21. Dan Dan Noodles * with chili minced pork
23. Szechuan Pork Dumpling* with roasted chili soy
50. Crispy Lamb Filets * with chili cumin
60. Chef's Ma Paul Tofu *** with chili minced pork
The tea smoked duck isn't spicy at all.
My recommendations are mostly based on trips to the 56th St. location, but I think the food is mostly the same as the 39th St. location's.
I'm sure you have plenty of good recommendations for more adventurous dishes in the linked thread. Here are some "safe" options:
If your wife likes spicy food, get the fantastic cucumber "salad" cold dish. Otherwise, the sauteed string beans are one of their best dishes. The sauteed baby bok choy, which is full of garlic, is also great. Some of the soups are relatively mild, too, such as the mixed vegetable and tofu soup. All the ones I've tried have been good. The uptown branch serves a sliced fish and mustard greens soup that can be great, but I don't see it listed on the midtown branch's menu, unless it's "fish filets." Also, I don't see the beef dishes with miso on the 39th St. branch's menu. One of them is moderately spicy, and both are good. Which branch are you going to?
There's a bunch of threads on this elsewhere. See: http://chowhound.chow.com/topics/738351
Personally my must-orders include mapo (ma paul) tofu (spicy/tingly with Szechuan peppercorns!), dan dan noodles, crispy lamb w/chili cumin, cucumbers in hot oil, fried pork dumpling. Can't go wrong!